FIG. 11 is a circuit diagram showing a conventional switching power supply described in JP-A-10-191625. This circuit contains a switching device 51, a starting power supply block 63, a control circuit 66, a conversion circuit 68, and an output voltage detecting circuit 67 as main elements.
The switching device 51 comprises an n-channel MOS. An input main terminal 56 is connected to the drain terminal of the switching device. The input main terminal 56 is further connected to the anode terminal of an input capacitor 57 and to the starting power supply block 63.
The conversion circuit 68 includes a coil 52, a regeneration diode 53, and a capacitor 54. The anode terminal of the regeneration diode 53 is grounded. The node between the cathode terminal of the diode 53 and the coil 52 is connected to the source terminal of the switching device 51. The node between the other terminal of the coil 52 and the anode terminal of the output capacitor 54 is connected to the output main terminal 55.
The control circuit 66 comprises a PWM pulse forming circuit 69, a comparator 62, resistors 64 and 65, and a triangular wave forming circuit 70. The triangular wave forming circuit 70 forms a triangular wave carrier signal at a constant frequency (e.g. 100 kHz). The PWM pulse forming circuit 69 is connected to the gate terminal of the switching device 51. The control circuit 66 uses the voltage between both terminals of a capacitor for control circuit power supply 60 as its power supply voltage, and controls ON/OFF of the switching device 51 according to the change in the potential difference between both terminals of the capacitor 60. The control circuit 66 also is connected to the starting power supply block 63 through a power supply switching block 61.
The output voltage detecting circuit 67 comprises a diode 58 and a Zener diode 59. Its terminal at the side of the Zener diode 59 is connected to the output main terminal 55, and its terminal at the side of the diode 58 is connected to one terminal of the capacitor for control circuit power supply 60, respectively.
This conventional switching power supply circuit is a power supply circuit of a step-down chopper system, in which a DC voltage applied to the input main terminal 56 is stepped down and outputted from the output main terminal 55.
FIG. 12 shows current/voltage waveforms at respective parts of the switching power supply in FIG. 11. The signs at the left of the waveforms in FIG. 12 correspond to the signs indicated at respective parts in FIG. 11. The power supply switching block 61 is closed so that the starting power supply block 63 is connected to the capacitor for control circuit power supply 60 until the control circuit 66 is started. When a voltage is applied to the input main terminal 56 first, a current flows from the starting power supply block 63 into the capacitor for control circuit power supply 60 through the power supply switching block 61, so that the voltage supplied to the control circuit 66 is increased.
The control circuit 66 operates when the supplied voltage exceeds a starting voltage of the control circuit 66. At this time, the output voltage Vout is 0 V. A triangular wave carrier signal voltage formed in the triangular wave forming circuit 70 and a voltage obtained by dividing the power supply voltage Vc of the control circuit 66 by the resistors 64 and 65 are compared by a comparator 62, and an output signal Vg indicated in FIG. 12 is supplied from the PWM pulse forming circuit 69 to the gate terminal of the switching device 51. The output signal Vg is ON for a certain duration. As described below, this duration is variable depending on the voltage between both terminals of the capacitor for control circuit power supply 60. When the output signal Vg is ON, the switching device 51 is ON, and a current Ip flowing in the switching device 51 flows into the coil 52. Next, when the switching device 51 is turned OFF by the output signal Vg of the control circuit 66, an electric energy accumulated in the coil 52 is supplied to the output through the regeneration diode 53.
If the voltage at the output main terminal 55 is increased to be greater than the sum of the puncture voltage Vz of the Zener diode 59, the forward voltage Vf of the diode 58, and the power supply voltage Vc of the control circuit 66 (Vz+Vf+Vc), when the switching device 51 is OFF, a current Ic flows from the output main terminal 55 into the capacitor for control circuit power supply 60 through the Zener diode 59 and the diode 58, so that information of the output voltage is fedback to the control circuit 66. When the power supply voltage Vc of the control circuit 66 becomes high enough, the supply switching block 61 switches so that the power supply voltage is supplied from the output main terminal 55 to the control circuit 66.
The triangular wave carrier signal voltage formed in the triangular wave forming circuit 70 and the voltage obtained by dividing the power supply voltage Vc of the control circuit 66 by the resistors 64 and 65 (or the power supply voltage Vc) are compared by the comparator 62, and on-duty of the switching device 51 in one triangular wave (1 carrier) is determined by the PWM pulse forming circuit 69, and then the pulse duration to be inputted to the switching device 51 is determined.
Thus, in the conventional switching power supply, by variably controlling the duty of the switching device 51, precision of the voltage at the output main terminal 55 is improved, and the output voltage Vout is kept constant.
In the conventional switching power supply circuit, pulse duration control system (PWM system) is used to improve the precision of the output voltage. Generally, the switching frequency fc is constant in this circuit, which is commonly 100 to 200 kHz. The PWM pulse forming circuit 69 determines the on-duty .delta. of the switching device 51, and allows it to operate at a constant frequency with minimum on-duty at the time of a light load.
However, when the above-mentioned conventional technology is used, the following problems occur. First, power is supplied to or consumed by the output main terminal uselessly because the switching device switches independently of the lightness or heaviness of the load. Second, switching loss is increased due to the relatively high switching frequency fc. In recent years, energy saving has been required from the point of energy or global environmental protection. Thus, a further reduction in power consumption and improvement in efficiency have been required in power supplies (primarily switching power supplies). However, because of the above two problems, a further reduction in power consumption and improvement in efficiency have been difficult to attain in a conventional control system.